Allene reactions with carbon dioxide

The concept of employing three components, all of which contribute one carbon atom to the final allene unit (see Scheme 5.4), is illustrated in its purest form by the reaction of carbon dioxide (165) with 2 equiv. of an alkylidenetriphenylphosphorane 166, the process very likely involving the generation in the first step of a ketene intermediate 167, which subsequently reacts with further 166 to yield the allene product 168 (Scheme 5.25) [66]. 

Allene carboxylic acids have been cyclized to butenolides with copper(II) chloride. Allene esters were converted to butenolides by treatment with acetic acid and LiBr. Cyclic carbonates can be prepared from allene alcohols using carbon dioxide and a palladium catalyst, and the reaction was accompanied by ary-lation when iodobenzene was added. Diene carboxylic acids have been cyclized using acetic acid and a palladium catalyst to form lactones that have an allylic acetate elsewhere in the molecule. With ketenes, carboxylic acids give anhydrides and acetic anhydride is prepared industrially in this manner [CH2=C=0 + MeC02H (MeC=0)20]. 

In the meantime, Aresta published some further work with rhodium catalysts. Reaction of two molecules of allene with carbon dioxide yielded dimethyl pyrone (compare Equation 11) besides polyallene and a small amount of oligomers [71]. As catalyst Rh(dppe)(n°-BPh ) was used. 

Dohring A, Jolly PM (1980) The palladium catalyzed reaction of carbon dioxide with allene. Tetrahedron Lett 21 3021-3024... 

In contrast, reaction of allene with carbon dioxide in the presence of palladium or rhodium complexes affords a mixture of pyrones and esters. 

Similar [2+2] cycloadducts are also obtained from 46 and benzophenone and fluorenone. The reaction of titana allenes with carbon dioxide, ketenes and isocyanates proceeds across the C=0 bonds of the heterocumulenes to give the [2+2] cycloadducts 48. ... 

Allenes of the type (171) have been prepared by cycloaddition of ynamines with carbon dioxide. The reaction with diethylaminopropyne is complete in one hour at —60 °C and the only contaminant is a small quantity of the aminocyclobutenone (172). 

Carbon dioxide instead of aldehydes can be involved in Ni(0)-promoted reductive coupling reactions (Equations (76) and (77) Scheme 90).434,434a 434c A stoichiometric amount of Ni(COD)2/DBU reacts with C02 and dienes, alkynes, or allenes to afford a metallacycle intermediate. This metallacycle reacts with organozinc compounds or aldehydes in one-pot to give carboxylic acid derivatives. As shown in Scheme 90, double carboxylation occurs in the presence of dimethylzinc, where the stereochemical outcome is opposite to that of the reaction with diphenylzinc. 

The carbonation of polymeric anions using carbon dioxide is one of the most useful and widely used functionalization reactions. However, there are special problems associated with the carbonation of polymeric organolithium compounds317 . For example, Wyman, Allen and Altares318) reported that the carbonation of poly-(styryl)lithium in benzene with gaseous carbon dioxide produced only a 60% yield of carboxylic acid the acid was contaminated with significant amounts of the corresponding ketone(dimer) and tertiary alcohol(trimer) as shown in Eq. (70). 

Anhydro-5-hydroxyoxazolium hydroxides lacking substituents at C(4) dimerize spontaneously by a process in which one molecule acts as an electrophile and the other as a nucleophile (Scheme 21). This accounts for the fact that dimeric products of this type are obtained by the action of dicyclohexylcarbodiimide on acylamino acids of the general formula R1C0NR2CH2C02H. Substituents at position 4 stabilize the mesoionic system the first compounds to be prepared were the acetyl derivatives (220) (B-49MI41800) and (221) (58Cl(L)46l) and much of the more recent work has been carried out with the relatively stable methyldiphenyl compound (222). This miinchnone decomposes above 115 °C to yield the allene (225) with loss of carbon dioxide. The mechanism proposed for this remarkable reaction (Scheme 22) involves valence isomerization to the ketene (223), which undergoes a 1,3-dipolar cycloaddition with the miinchnone. The product loses carbon dioxide to form a new betaine (224), which collapses to the allene as shown. 

Although the sp-hybridized carbon atoms are commonly found in acetylenes, nitriles, allenes, carbon dioxide, etc., heavy group-14 element analogs of these compounds are very rare [16, 17]. The first silicon compound with a formal s/i-silicon atom, trisilaallene 12, was synthesized as a dark green solid using two-step reactions from stable dialkylsilylene 13 [18] in overall 42% yield (Scheme 5) [19]. Trisilaallene 12 is sensitive to air but thermally rather stable, with a melting point of 198-200 °C. 

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